This project will prospectively investigate the role of cerebral hemodynamic factors in the pathogenesis of stroke in patients with symptomatic carotid artery occlusion using positron emission tomography (PET). Patients who do not anticipate cerebrovascular surgery will undergo PET studies and then be followed to determine if the subsequent occurrence of stroke is related to the PET measurements. The primary objective of this project is to test the hypothesis that increased oxygen extraction in the cerebral hemisphere distal to symptomatic carotid artery occlusion is an independent predictor of the subsequent risk of stroke in medically treated patients. The secondary objectives are (1) to determine other predictive factors for stroke in this population, and (2) to determine the temporal evolution of cerebral hemodynamic abnormalities measured by PET. These objectives will be accomplished by the use of positron emitting radiotracers to measure regional cerebral blood flow (rCBF), regional cerebral blood volume (rCBV), and regional cerebral oxygen extraction fraction (rOEF) in patients with symptomatic internal carotid artery occlusion. Information on potential epidemiologic, clinical and angiographic risk factors will be collected at the time of PET. Patients will be followed to determine the subsequent occurrence of stroke. We will then do multivariate analysis to determine the independent predictors for stroke in this sample. A subgroup (25% of patients) will be brought back for yearly repeat PET scans to determine the temporal evolution of the PET findings.